Events on the battlefield are becoming more and more complex and take place at increasing speed. It is necessary to make decisions in fractions of seconds, even though the consequences of errors are enormous. It is therefore necessary to convey relevant information to the decision makers succinctly and easily comprehensible in order to make rapid and error-free decisions possible. Among the most important information from the battlefield actually is the determination whether or not an object or a person is hostile.
Very advanced systems have been developed for the identification of aircraft and some other large devices capable of being used on the battlefield. Such systems are called IFF--(Identification-Friend or Foe) systems. But the problem of identifying individual soldiers taking part in the battle has up to now remained unsolved to a large degree, but is all the more pressing, since parties to a conflict can no longer be differentiated by the clothing of the soldiers and a clear definition of the front. It is therefore necessary to equip ground troops with systems which, prior to initiating an attack on a target, provide them with assurance regarding its identity.
A disadvantageous consequence of the increasing complexity of the technical systems employed on the battlefield is the increasing burdening of the individual soldier by the size and weight of the equipment which must be carried. For example, in some cases the weight of the equipment already has negative effects regarding the availability and flexibility of the individual soldier, so that an IFF-system should not be allowed to considerably increase the burden of the soldier with equipment. Since in modern concepts of fighting the soldiers often do not operate in larger groups, the IFF-system must make possible the identification not only of groups, but also of individual soldiers. In fact, the solitary soldiers, not distributed in groups on the battlefield, need the protection from the effects of the weapons of their own party the most.
Furthermore, combat events are conceivable today wherein individuals of many different cultures, languages and races fight as allies against a foe who is just as heterogeneous. Identification by means of appearance or language therefore contains high risks. Combat in modern war usually takes place at night and constitutes a considerable additional difficulty of identification by sight, furthermore, time also plays an important role. Therefore an IFF-system must provide the relevant information extremely rapidly. To be useful for ground troops, IFF-devices therefore must combine extreme sensitivity and accuracy as their properties and must be easy to carry or to transport.
In general, lasers emit a strongly collimated, quasi-monochromatic light beam. Semiconductor lasers made of various materials include a light spectrum extending from the ultraviolet to the infrared range in their emissions. Lasers available at this time are very dependable and become smaller and more efficient practically from one day to the next. For example, the semiconductor chip of a GaAs diode laser has extremely small dimensions, comparable with the size of a pinhead (without taking its power supply into consideration). Semiconductor lasers can be embodied to emit in pulses or continuously. With an appropriate power supply, modern gain-producing laser media make possible the generation of pulses of a length of a few nanoseconds by means of a laser embodied to emit continuously.
Semiconductor lasers emitting in the infrared range are ideally suited for applications in IFF-systems. Light beams emitted by infrared diode lasers cannot be detected by the unaided eye. Such light beams can only be detected by using special viewing aids, such as night-vision goggles.
Using an optical device which must be appropriately designed, the light emitted by an infrared laser diode can be collimated into a tightly bundled beam, which is ideally suited for illuminating point targets. Accordingly, collimated light beams from infrared laser diodes have a beam diameter of approximately 5 cm at a distance of 100 m, so that it is possible to illuminate practically every point target in a very accurately isolated manner. Furthermore, because of the narrow beam angle of the laser beam used, there is great security from countermeasures.
Laser diodes which presently are commercially available already have outstanding efficiency, more than 70% of the electrical output provided the semiconductor laser are converted into light output. Among other things, lasers have the exceptional quality of emitting light in the form of a single, narrow and almost diffraction-limited beam. In comparison with this, a 100 Watt light bulb may radiate a considerably greater light output than a comparatively low output laser, but the light radiated by the glowing wire is spatially and chronologically incoherent, because of which it has, on the one hand, a broad optical spectrum, but on the other hand it is radiated over a large spatial angle in spite of the relatively large surface of the glowing wire.
A high-quality lens can completely capture the light radiated by a laser and can focus nearly its entire optical output on an almost diffraction-limited spot of a diameter within an order of magnitude of a few millimeters. The optical power densities which can be achieved by focusing the light of a continuously emitting diode laser of moderate output with a light output of, for example 25 mW, are more than 50 kW/cm.sup.2. The power density at an oxyacetylene flame can be cited for comparison, which is approximately 1 kW/cm.sup.2.
An IFF-system requires a very good detection system. In this sense the basis for an IFF-system is an extremely sensitive detection system, which also operates dependably under difficult light conditions on open ground with bushes.
In order to be able to optimally detect the emission of the already described infrared laser, corresponding infrared detectors would be very important in a detection system for the IFF-system. So-called PIN photodiodes consist of a p- as well as an n-doped zone, both of which are separated by a zone of non-doped intrinsic material. Usually these photodiodes are designed in such a way that the major part of the incident radiation is absorbed in the non-doped intrinsic region, by means of which it is assured that all charged particles generated by the light are caught in the internal electrical field of the photodiode and add to the photo-electric current. Since the non-doped intrinsic zone separates the positive and negative volume charge, a PIN photodiode furthermore has a lesser capacitance in comparison with a PN photodiode, because of which the reaction time of a detection system becomes very short. A short reaction time is ideal in connection with IFF-systems based on the transmission of high rates of short pulses.
Because of their excellent stability in respect to environmental effects as well as their capability of being used over a wide range of temperatures, PIN photodiodes are suitable for use with IFF-systems employed in the open. A further important point for their application in connection with such systems is the minimal effects of sunlight on the detection system. Several manufactures produce photodiodes with special filters applied directly to the surface of the chip, which suppress undesired spectral ranges. These filters can be used to eliminate a large part of the light of the sun or other interfering sources existing in the vicinity, such as that of street lights or moving lights. The stability of modern PIN photodiodes over a long time assures that no measurable decrease of their sensitivity will occur over their lifetime.
Since pulsed lasers have inherent jitter (the period between two pulses is not constant), fluctuations of the pulse output (typically several percent), as well as large fluctuations in the pulse length (often around 50%), there are only limited possibilities for detection and achieving high sensitivity. Therefore the use of continuously emitting lasers is preferred for IFF-systems. As can be seen from the application entitled "Continuous Wave Laser Battlefield Simulation System" with U.S. Ser. No. 08/565,960, which is connected with this application and is cited as a reference here, permits the use of PPM (pulse position modulation) and PCM (pulse code modulation) as a method for coding "lock-in" and other detection methods for achieving high sensitivity. The light source emits a signal of a chronological progression, which is assured by the accuracy of quartz oscillators, wherein the receiver is also equipped with quartz oscillators of corresponding frequencies. Sensitivities, which cannot be achieved with pulsed lasers, are achieved by using continuously emitting lasers. In connection with the invention described here, these are in the range of nanoWatts. An extremely long effective range is made possible by means of this, and the realization of a system is permitted, which assures absolute safety even for unprotected eyes, and whose efficiency is not considerably diminished by leaves, fog and rain.
The already mentioned "Continuous Wave Laser Battlefield Simulation System" (SIMLAS) of applicant uses laser and light emitting diodes (LEDs) for simulating weapons including, but not limited to rifles, pistols, hand grenades, tanks and land mines. In every case the weapon is normally used by soldiers, and their effect is represented by a light beam as realistically as possible, be it that of a rifle or pistol projectile, an exploding hand grenade, etc. All participants in such an exercise (both persons and objects, such as tanks, aircraft, all-terrain vehicles, trucks, etc.) are equipped with detectors which register a possible effect of a weapon on the participant (for example a direct hit or near miss).
For example, the laser light source transmits a narrow infrared light beam, whose divergence is only 0.2 mrad. From this a beam diameter of only approximately 4 cm results after 100 m, because of which this laser can be used as an aiming aid together with night vision devices. However, the divergence can also lie between 0.1 and 5 mrad, or between 0.2 and 2 mrad, for example. Assembly and alignment of the laser can be made so permanently, that the system meets military standards in respect to shock resistance. The laser light beam being used with the SIMLAS is modulated by a code in such a way that both the acting person and the type of weapon used can be identified. By using signals in accordance with well-defined rules, all phases of training can be simulated by SIMLAS, including: a., registration of direct hits, near misses, injuries, putting out of action, etc., b., identification of troops and their status; c., registration of all events, including time and acting person; and d., compilation of all action-related data of individual persons or groups. To perform these tasks, continuously emitting laser diodes are used with SIMLAS. The light beam is coded in PCM and PPM, since these types of modulation combine great exactness with high sensitivity and immunity against interference signals and noise. The function of SIMLAS represents the technical basis of the IFF-system in accordance with the instant invention.